This application claims the benefit of European Application No. 01403213.0 Filed Dec. 10, 2001.
1. Field of the Invention
The invention relates generally to fiber lasers. In particular, the invention relates to multi-wavelength fiber lasers.
2. Background Art
Many advanced communication system rely upon wavelength division multiplexing (WDM) over an optical fiber communication link or network. An optical fiber has a very large transmission bandwidth in specific windows. For silica fibers, these windows are near 850 nm, 1310 nm, and 1550 nm. In a WDM system, respective data signals are impressed upon multiple optical carriers of specific and different wavelengths, and all the modulated carriers are coupled into the same optical fiber. The number of channels in a dense WDM system may be very large, for example, 40 channels spaced by 100 GHz in frequency, which is about 0.8 nm in wavelength for a 1550 nm signal wavelength. The technology for fabricating the various components of such a WDM communication system is well in hand.
However, during development or fabrication, these components need to be tested over their usable wavelength band. This testing has presented a problem because of the large number of discrete channels. The conventional approach mimics a WDM transmitter in which a plurality of distributed feedback (DFB) lasers are fabricated each having an emission wavelength corresponding to one of the wavelengths of the ITU grid defining the WDM wavelengths. While the separate DFB lasers can be individually modulated, much of the testing does not require data modulation. For many tests, a flat and unmodulated spectrum of channels is sufficient to test system for channel response. Additionally, forty or more DFB lasers present a significant cost for one piece of laboratory equipment.
Several suggestions have been made for a multi-wavelength laser source based on a ring laser incorporating an erbium-doped fiber amplifier (EDFA) and various types of gratings to define the wavelength spectrum of the lasing light signals. The EDFA provides optical gain based upon long lived electronic energy levels of a rare-earth ion such as erbium doped into the fiber. However, the energy levels are discrete and narrow, thereby resulting in a non-flat gain spectrum and often necessitating multiple pump wavelengths.
The invention includes a multi-wavelength Raman fiber laser in which a Raman fiber is incorporated into an optical ring. A periodic filter is placed on the ring to define the lasing wavelengths. Optical pump power is coupled into the ring, and Raman amplified light is coupled out of the ring. An optical isolator allows lasing in only one direction of the ring and is preferably coordinated with the directions of the couplers for pump power and output signals.
A bandpass filter may be needed to define the limits of the amplification and filtered band.
A frequency shifter may be needed to prevent amplification from being concentrated in only one or two wavelengths by breaking the normal mode structure and preventing competition between modes. The frequency shift may be equal to spacings of the periodic filter or may be substantially less so that several round trips occur before an adjacent channel is encountered. An acousto-optical modulator may serve as the frequency shifter.
The output coupler may be a wavelength division multiplexer selectively coupling out only selected wavelengths. Alternatively, the output coupler may be less selective to frequency but a fiber Bragg grating may be written onto the output fiber to reflect the pump wavelength while passing the amplified wavelengths.